In early August 2015, Columbia’s national health service began to collect what would add up over the year that followed to about 105,000 reports of suspected Zika virus infections, with close to 20,000 of those among pregnant women.

Six months later, health officials began to collect reports of a steeply climbing number of babies born with microcephaly, with the total, among 476 newborns, stillborn infants, and lost pregnancies, in the next year adding up to four times the number of fetuses and infants affected by the disabling and disfiguring neurological birth defect than the year before. Six months after the peak of Zika virus infection reports, reports of microcephaly peaked.

The data, researchers who examined it write in the most recent U.S. Centers for Disease Control and Prevention Morbidity and Mortality Weekly Report, indicates that the danger of Zika infection in pregnant women causing the birth defect is greatest in the first and the beginning of the second trimesters of pregnancies. But, the researchers add, that information falls far short of sizing up the risks the vector-born and sexually transmitted virus poses to fetuses, as it does not include babies born with neurological damage caused by the virus born without visible birth defects.

And the impact of the virus on babies born to mothers infected during pregnancy in Columbia appears to fall far short of the impacts in Brazil, where health officials saw nine times the numbers of microcephalic newborns in the year after Zika virus began to spread there. The authors of the MMWR report point to several reasons, including that health officials in Brazil were unaware of the danger the virus would pose until its impacts began to appear, while health officials in Columbia, alerted by Brazil’s experience, cautioned women potentially exposed to the virus to postpone pregnancy for at least six months. In addition, the proportion of the population overall exposed to the virus in Columbia is smaller; as many as 75 percent live at altitudes out of the reach of the mosquito that spreads the disease. People living in those areas who did become infected were likely exposed through travel or sexual transmission, the researchers note.

Finally, surveillance was limited in a number of ways — from adequate active surveillance of the spread of the virus, to timely available testing to confirm its presence, to a consistent definition and surveillance of birth defects. The data gives a glimpse of the impacts of Zika virus, but ongoing monitoring is critical the authors say, to measure the effects both of infection, and of efforts to prevent it.